1. Field of the Invention
The present invention relates to jitter detecting, and more particularly, to a method and an apparatus detecting a jitter amount using an analog-digital conversion circuit.
2. Description of the Related Art
Generally, in a technique to convert an analog signal into a digital signal, the analog signal is binarized through a comparator, and a phase locked loop is applied to the binarized signal to make a system clock. Data is processed using the binarized signal and the system clock. Here, a synchronism of the analog signal does not correspond accurately with that of the system clock, and a little phase difference exists, which is called a jitter.
As an analog method of detecting a jitter becomes generalized, a method of binarizing, using a converted digital signal after converting the analog signal into the digital signal through the analog-digital converting process, is used instead of binarizing the analog signal by directly using a comparator. Since the method of detecting a jitter used in the latter case detects a jitter value using a signal of one sampling point around the edge of the analog signal, the jitter value is not detected correctly if the analog signal is small or an error signal exists.
FIG. 1 is a block diagram of a general digital system including a jitter detecting circuit, wherein the system includes an A/D converter 100, a DC offset eliminator 110, a jitter detector 120, and a digital phase locked loop (PLL) circuit 130. The A/D converter 100 converts an analog input signal into a digital signal, and generates digital data corresponding to the digital signal. The DC offset eliminator 110 eliminates a direct current voltage offset from the digital data. The jitter detector 120 detects a jitter value from the offset-eliminated signal. The digital PLL circuit 130 generates a locked signal from the offset-eliminated signal, and the locked signal is generated as the system clock. As an example, Korean Patent P1998-0049210, published on Nov. 17, 1998, applied by the same inventor, discloses a method of calculating a jitter value from analog/digital converted digital data. However, in such a case, if an analog signal with small amplitude enters the system, the analog to digital converted value itself is small, and the jitter amount detected from the value is also small. In the case of an input signal having an error, in which the amplitude of the signal greatly varies, there can be an error in the operation of the circuit itself.